An Inexpensive APRS Weather Station

Accuracy Counts!

BACKGROUND

The purpose of this page is to really emphasize how important it is to
do all you can to make your weather data as accurate as possible. As
amateurs, we are not beholding to any standards other than the ones we
set for ourselves. Our credibility for providing accurate weather data
to other people depends on how accurate our instruments are and how well
we mount them and care for them. This page describes all the nasties
that can affect your good data so you know the limitations and strengths
of the hardware and software and what you can do to keep things as
accurate as possible.

THE HARDWARE

WIND SPEED:

We checked out four weather sensors in a calibrated wind tunnel to see
how well they performed. That data is the basis of our wind speed
calibration values. Above 10MPH, the sensor units agreed with the wind
tunnel reading within +/-2%. Variances between units were less than 5%.
Below 10MPH the unit starts to read low, falling off to about 10% low at
5MPH and 30% low at 3.5MPH.

WIND DIRECTION:

The basic 1Wire weather station hardware reads one of 16 equally-spaced
directions for each five second sample. This is lower resolution than
most users want. Short-term wind direction is highly variable due to
atmospheric turbulence and when you take a bunch of samples and average
them together, the variation is averaged out and you are left with a
representative value of wind direction.

TEMPERATURE:

The weather station temperature sensor is supposed to be accurate to
within +/0.5 degrees C over the 0-70 degree C range. Below 0 degrees C,
it is nonlinear, but for temperatures in the lower United States, errors
will be less than 2 degrees. Because the temperature sensor is inside
the outdoor plastic unit in a sealed environment, it can become hotter
than ambient when the sun is shining directly on the housing. This can
result in significant temperature error. We ran a one-week test using
the standard outdoor units, a homebrew radiation shield, and a
commercial radiation shield and compared them. Our worst case error was
+16 degrees F on a calm sunny day. On our web page is a set of plans
and the directions to construct an inexpensive radiation shield which
will reduce this error by a factor of four in light winds and more in
higher winds. Dallas is considering an external temperature sensor that
could be placed in a protected environment. If that becomes available,
we will announce it and add software support for it as soon as
practical.

CABLING:

One of the greatest things about this weather station is that all the
data coming down the cable is already digital. Cable length does not
impact reading accuracy at all.

THE FIRMWARE

WIND SPEED:

Since we know the errors introduced at low wind speeds, we can
compensate for it in firmware. It is on our ďto doĒ list. Watch the
release notes to know when we add this feature.

WIND DIRECTION:

In our case, there are three kinds of noise in the wind direction data.
The first is variation in wind direction (turbulence). The second is
sampling error due to the limited number of data points available. With
averaging we can still claim accuracy within six degrees for the
averaged data. See the FAQ page for lots of discussion about this. The
third source of error we canít control is an offset error when you first
calibrate the wind direction sensors. That is a single sample, and if
you do point perfectly accurate, we could still be off up to 11.25
degrees. That one hurts, but there isnít much we can do about it.
Fortunately, this is the measurement most tolerant of errors.

TEMPERATURE:

There is little the firmware can do to compensate for the solar heating
of the outdoor unit. The best approach here is to use a radiation shield
to keep the solar radiation off of the sensor and to dissipate the heat
due to the direct solar radiation. The home-made radiation shield on
our web page will reduce these errors by a factor of about four, so it
is a good option for anyone who doesnít mind building something.

CABLING:

All data coming down the cable is verified with one of two types of
CRCs, so that erroneous data will be eliminated. There is quite a bit
of code not only checking the data, but making sure bad or missing data
does not mess up the averaging routines. One of the (soon to be)
weather display screens will show cumulative communication error counts.
Hopefully these will always read 0, but occasional errors may be
possible. If you see lots of errors, then you have some kind of cabling
problem that needs addressing.

YOUR PART

Your part in making sure accurate wind data get out is to make sure the
sensor unit is in a good place to get a good sample of the weather.
Below is a good description of what is needed by Ray McKnight WB3ABN
RMcKnight@PACNORWEST.USCG.mil

STANDARDS FOR WEATHER SENSOR INSTALLATION

To aid those Amateurs who install home/amateur weather stations, I have
extracted the various recommendations regarding the proper methods for
installing the various sensors. These recommendations are contained in
FMC-S4-1994, "Federal Standards for Siting Meteorological Sensors at
Airports", and meet both FAA and NWT guidelines.

In following these standards you will ensure your weather station is
providing the most accurate readings possible. Obviously, you will need to
adapt these guidelines to your particular installation and may not be able
to obtain the exact results listed. These standards, at minimum, should
provide guidance in the mechanics involved in obtaining accurate sensor
readings.

Atmospheric pressure:
Should usually be installed in a weatherproof building or enclosure (most
home units include this sensor in the main display). Avoid areas that are
affected by pressure changes caused by compression due to closing doors in
small rooms, avoid jarring, vibration and rapid temperature changes. Avoid
direct sunlight, drafts from open windows, and air currents from heating or
cooling systems. Air flowing over a sensor can cause artificially low
readings. It should be at least 3 feet above ground level, but less than
100 feet above average terrain. If installed in a closed room, venting to
the outside may be necessary.

Wind speed and direction:
Should be oriented to TRUE NORTH, not magnetic north! BE AWARE that in
some
areas true north may vary by 15 degrees or more from a raw compass reading.
Call the nearest Coast Guard unit for your local magnetic variation. The
site should be relatively level but small gradual slopes are OK. This
sensor should mounted 30-33 feet above average terrain, EXCEPT it should be
at least 15 ft above any obstruction within 500 feet. If installed on a
tower, it should be at the top, but a side bracket can be used and should
be at least 3 feet away from the tower side. Towers should be of an open
design to permit free air flow. This will be the hardest sensor to install
at a home location to meet the intended guidelines. Don't cut down your
neighbors trees to reduce obstructions! Achieve the best compromise
possible. You should also check this sensor at least monthly to ensure free
movement of the wind cups and vane, as well as proper orientation to true
north.

Temperature and dew point:
Should be at 5 feet (+/- 1ft) above ground level, or 2 feet above average
maximum snow depth. It must be adequately ventilated but needs protection
from direct sunlight, earth radiated heat, etc. Also it should not be
influenced by artificial conditions like concrete or blacktop, heat
radiating from buildings, heating/cooling exhausts, etc. Avoid placing
above house roofs or near windows, doors or roof eaves. Vegetation near a
sensor should be clipped to 10 inch height or less.

Lightning detector:
Install according to manufacturers directions. Metal obstructions should
be
no closer than 2 times their height.

Precipitation accumulation:
Must be level (measure the upper rim or orifice). Surrounding terrain
should be as flat as possible. It should be as close as possible to ground
level, and free from surround obstructions (no closer than twice their
height). Avoid hard surfaces which may allow splashing into sensor. To
avoid losses due to wind, install an "alter-type" wind shield. In areas
with rain or sub-freezing temperatures, it must be heated to measure snow
or
freezing rain. In home units, this is usually the least accurate sensor.
Check periodically to ensure sensor is free of debris.

Consult the manufacturer for guidelines in extending cables. You should
also keep RF sources as far away as possible from sensor cables and the
processor unit. Cable shielding may be necessary. You will also need to
set your corrected sea level pressure. Call the nearest airport and ask
the
control tower for CORRECTED SEA LEVEL PRESSURE, not airfield local
altimeter
pressure.

Hope this helps you get the most accurate readings possible. NWS DOES use
APRS reported weather, so we should strive to be as accurate as we can!